[Dovecot] Converting from qpopper mbox to dovecot maildir

[Joseph Tam]

Dimos Alevizos <dalevizo at otenet.gr> wrote:

> We recently completed the migration in our company from qpopper/mbox to
> dovecot/mbox using only pop3_reuse_xuidl = yes and while everything went much
> easier than we feared we came across an interesting problem that you might want
> to pay attention to.
> 
> We use dovecot to provide both pop3 and imap (directly and via webmail) access
> to our client's mailboxes and we noticed that when a client moves a mail to a
> different folder and then back into INBOX (say he deleted it by mistake and
> moves it back), dovecot inserts the same X-UIDL into it's cache, thus creating
> duplicate entries.
> 
> When a pop3 client sees this (at least outlook in our case) it think's that
> there's something wrong with the server's X-UIDL handling and re-downloads the
> duplicate messages. EVERY time the user checks his mails.

I haven't heard any user complaint about this, but
perhaps nobody tried this before.

I recreated this situation (copy old qpopper message INBOX -> tmpbox ->
INBOX) and produced two messages with identical X-UIDL headers, but
according to the Wikipedia entry for POP3:

 	Comparison with IMAP

 	Clients that leave mail on servers generally use the UIDL command
 	to get the current association of message-numbers to message
 	identified by its unique identifier. The unique identifier
 	is arbitrary, and might be repeated if the mailbox contains
 	identical messages.

This is echoed in RFC1939 (page 11-12)

 	While it is generally preferable for server implementations
 	to store arbitrarily assigned unique-ids in the maildrop,
 	this specification is intended to permit unique-ids to be
 	calculated as a hash of the message.  Clients should be able
 	to handle a situation where two identical copies of a
 	message in a maildrop have the same unique-id.

So POP3 UIDLs are not guaranteed to be unique, especially if the messages
are the same.  Perhaps some mail readers are making unwarranted assumptions
about the uniqueness of UIDLs:

 	http://www.eggheadcafe.com/software/aspnet/33485042/duplicate-messageiduidlleave-on-serverrepeated-downloads-of-em.aspx
 	https://www.ritlabs.com/bt/view.php?id=3599

Joseph Tam <jtam.home at gmail.com>